Here's a direct quote from Graham Bell's 2 stroke performance tuning manual discussing what the ideal dimensions of an expansion chamber should be -
"The length of the diffuser is determined by the diameter to which it expands, which should normally be 2.5 times the exhaust port diameter. If you have room on the bike, and you wish to experiment, you may be able to spread the power range by making the diffuser taper to 2.7 - 2.9 times the port diameter."
On a stock cylinder the exhaust port is very small. This means that ideally, a smaller diameter expansion chamber (such as the one on a biturbo) will perform better across the entire throttle range.
On a kitted bike, the exhaust port is much, much larger. Because of this, a larger expansion chamber can be efficiently utilized, such as the treats pipe.
From a strictly performance standpoint I believe the biturbo would perform better on a stock cylinder, however I can vouch that the quality of it is complete crap. The silencer on my old biturbo rattled apart and the stinger inside literally broke in half after about 1500 miles.
If you're looking for maximum performance go for the biturbo. If you're looking for a future proof pipe that will last and work well with a kit if you so choose to upgrade, grab the treats pipe. (edited)